A plane emission laser has various features. For instance, the laser is capable of emitting laser light substantially perpendicularly with respect to a substrate surface, integrating plural laser elements two-dimensionally (configuring into a two-dimensional array), and emitting coherent light from each of the laser elements in parallel. In view of these features, the plane emission laser is expected to be applied in various technical fields such as a storage field, a communications field, and an information processing field; and research and development of the plane emission laser has been progressed.
The plane emission laser generally and basically adopts a structure, wherein an active layer for generating light by carrier impregnation is vertically sandwiched between multilayer reflection mirrors (DBR mirrors). In addition to the above, a two-dimensional photonic crystal is used, as disclosed in e.g. patent literature 1. Generally, a photonic crystal is an optical element internally provided with a periodic refractive index distribution substantially equal to or smaller than a wavelength of light. Examples of the photonic crystal are a three-dimensional photonic crystal having a three-dimensional refractive index distribution, and a two-dimensional photonic crystal having a two-dimensional refractive index distribution. Similarly to a phenomenon that a band gap is formed in a semiconductor, when electrons (an electron wave) are subjected to Bragg reflection by a periodic potential of atomic nuclei, the photonic crystal has a feature that a band gap (a photonic band gap) with respect to light is formed, when a lightwave is subjected to Bragg reflection depending on a periodic refractive index distribution. Since light itself is not allowed to reside in the photonic band gap, control of light using the photonic crystal is expected.
The two-dimensional photonic crystal plane emission laser disclosed in patent literature 1 comprises a substrate, an n-InP layer formed on a principal plane of the substrate and serving as a lower clad layer, an active layer formed on the n-InP layer and for generating light by carrier impregnation; a p-InP layer formed on the active layer and serving as an upper clad layer, a first electrode and a second electrode formed on the p-InP layer and the other principal plane of the substrate, and a two-dimensional photonic crystal formed near the active layer and within the n-InP layer. The plane emission laser having the above construction is operated in such a manner that upon application of a voltage of a predetermined value or more between the first and the second electrodes, the active layer emits light, and the light is incident from the active layer into the two-dimensional photonic crystal. The light incident into the two-dimensional photonic crystal is amplified by resonance of light having a wavelength equal to the lattice constant of the two-dimensional photonic crystal for oscillation, whereby laser light is emitted in a plane direction.
FIG. 32 is a plan view showing a structure of a two-dimensional photonic crystal to be used in a two-dimensional photonic crystal plane emission laser. FIGS. 33A through 36B are diagrams for describing resonance modes. FIGS. 33A and 33B show A mode, FIGS. 34A and 34B show B mode, FIGS. 35A and 35B show C mode, and FIGS. 36A and 36B show D mode. FIGS. 33A, 34A, 35A, and 36A are diagrams each showing a near-field electric field distribution obtained by solving a Maxwell equation, wherein each arrow shows a direction and a magnitude of electric field. FIGS. 33B, 34B, 35B, and 36B are conceptual diagrams corresponding to FIGS. 33A, 34A, 35A, and 36A, respectively, wherein each arrow shows an electric field direction at each lattice point in ±X direction and ±Y direction. FIG. 37 is a diagram showing how a two-dimensional photonic crystal plane emission laser emits planar light.
In describing the resonance of the two-dimensional photonic crystal in detail, for instance, as shown in FIG. 32, let it be assumed that a two-dimensional photonic crystal 100 includes, in a first medium 101 having a first refractive index, a second medium 102 (lattice points) having a second refractive index different from the first refractive index and constituted of square lattices, wherein the cylindrical column-shaped lattice points 102 are arranged in two directions orthogonal to each with a predetermined period (a lattice interval or a lattice constant) “a”. If light L having an in-medium wavelength λ equal to the lattice interval “a” is propagated in Γ-X direction as a side direction of the square lattice, the light L is diffracted at one of the lattice points 102. Out of the diffraction light L, only the light L which is diffracted in the direction of 0 degree, ±90 degrees, and 180 degrees with respect to the propagating direction of light L satisfies the Bragg condition (2×a×sin θ=m×λ (m=0, ±1, . . . )). There are other lattice points 102 in the propagating direction of light L diffracted in the direction of 0 degree, ±90 degrees, and 180 degrees. Accordingly, the diffraction light L is diffracted again in the direction of 0 degree, ±90 degrees, and 180 degrees with respect to the propagating direction. Then, the light L propagating in Γ-X direction is returned to the original lattice point 102 by repeating the aforementioned diffraction once or plural times. Thus, the two-dimensional photonic crystal 100 is resonated. In using a square lattice, there also exists a diagonal direction i.e. Γ-M direction, as a typical direction. Accordingly, it is possible to oscillate the two-dimensional photonic crystal 100 using Γ-M direction, based on the same principle as described above.
There are four modes as resonance modes at which light is oscillatable by resonance. If a xy coordinate system is defined on a two-dimensional plane of the two-dimensional photonic crystal 100, with an arbitrary lattice point 102 being defined as a coordinate origin, a standing wave to be formed by resonance at the lattice point 102 has the following features in each of the modes, as shown in FIGS. 33A through 36B.
In the first mode (hereinafter, called as “A mode”), a primary component Ey of standing wave, which is composed of a y-direction component of electric field to be formed in x-axis direction, starts at +sin (see A1), and a primary component Ex of standing wave, which is composed of an x-direction component of electric field to be formed in y-axis direction, starts at sin (see A2). As a result, the electric field distribution is as shown in FIG. 33A.
In the second mode (hereinafter, called as “B mode”), a primary component Ey of standing wave, which is composed of a y-direction component of electric field to be formed in x-axis direction, starts at +sin (see B1), and a primary component Ex of standing wave, which is composed of an x-direction component of electric field to be formed in y-axis direction, starts at +sin (see B2). As a result, the electric field distribution is as shown in FIG. 34A.
In the third mode (hereinafter, called as “C mode”), a primary component Ey of standing wave, which is composed of a y-direction component of electric field to be formed in x-axis direction, starts at +cos (see C1), and a primary component Ex of standing wave, which is composed of an x-direction component of electric field to be formed in y-axis direction, starts at +cos (see C2). As a result, the electric field distribution is as shown in FIG. 35A.
In the fourth mode (hereinafter, called as “D mode”), a primary component Ey of standing wave, which is composed of a y-direction component of electric field to be formed in x-axis direction, starts at +cos (see D1), and a primary component Ex of standing wave, which is composed of a z-direction component of electric field to be formed in y-axis direction, starts at −cos (see D2). As a result, the electric field distribution is as shown in FIG. 36A.
As is obvious from FIG. 35A and FIG. 36A, the electric field distributions are identical to each other, when one of the electric field distributions is rotated by 90 degrees. Accordingly, C mode and D mode are in a relation of degeneration.
The Bragg condition is also satisfied in a direction perpendicular to a plane of the two-dimensional photonic crystal 100. Accordingly, laser light is emitted in the direction perpendicular to the crystal plane in a state that in-plane electric field distributions are considered. In A mode and B mode, the electric field distribution is an odd function with respect to the lattices of the two-dimensional photonic crystal 100. Accordingly, diffraction waves in the perpendicular direction cancel with each other (generate cancellative coherence). As a result, theoretically, laser light is not emitted in the direction perpendicular to the plane of the two-dimensional photonic crystal 100. On the other hand, in C mode and D mode, the electric field distribution is an even function with respect to the lattices of the two-dimensional photonic crystal 100. Accordingly, laser light is emitted, without generating cancellative coherence. Observing the performance of a crystal-based resonator capable of trapping waves, the performance of the resonator is high with respect to A mode and B mode, and low with respect to C mode and D mode. As a result, the two-dimensional photonic crystal plane emission laser having the two-dimensional photonic crystal 100 shown in FIG. 32 has a property that the laser is most likely to oscillate in A mode, second most likely to oscillate in B mode, and hardly likely to oscillate in C mode and D mode.
In an actual device, the lattice number (the periodic number) of the two-dimensional photonic crystal 100 is definite. Accordingly, as shown in FIG. 33B, in A mode, cancellation of electric field components which invokes cancellative coherence is imperfect in electric fields E1, E2; E3, E4 corresponding to the opposing sides of a peripheral portion of the two-dimensional photonic crystal 100, and cancellative coherence in a direction perpendicular to a crystal plane is imperfect at an end portion of the two-dimensional photonic crystal 100. As a result, laser light may leak in the direction perpendicular to the crystal plane even in A mode, and the two-dimensional photonic crystal plane emission laser having the two-dimensional photonic crystal 100 shown in FIG. 32 emits planar light in annular shape (a ring-like shape or a doughnut shape), as shown in FIG. 37.
As a method for improving the performance of a two-dimensional photonic crystal-based resonator in the two-dimensional photonic crystal plane emission laser having the above arrangement, there are proposed firstly, a method of forming a two-dimensional photonic crystal near an active layer, secondly, a method of increasing a refractive index difference between a first medium and a second medium for forming a two-dimensional refractive index distribution of a two-dimensional photonic crystal, and thirdly, a method of increasing the lattice number (the periodic number) of a two-dimensional photonic crystal.
Generally, in an actual device, there is a limit in forming a two-dimensional photonic crystal near an active layer, in view of electrical constraints or thermal constraints. There is also a limit in increasing the refractive index difference in view of material constraints. In view of the above, generally, the third method is adopted.
In the third method, however, increasing the lattice number (the periodic number) in order to improve the performance of a resonator may increase (widen) the light emission area or the element size of a laser element. In view of the above, it is impossible to control (design) the performance of a resonator, and the light emission area or the element size independently of each other. Further, even if the lattice number (the periodic number) is increased, since a two-dimensional photonic crystal has an end portion, light cannot be completely trapped within the crystal. Accordingly, leaking light becomes a light loss for the resonator. In the case where plural laser elements are formed into an array, crosstalk may be generated between the laser elements depending on a leak direction.
There are proposed technologies disclosed in e.g. patent literature 2 and patent literature 3 to improve the performance of a resonator without increasing the lattice number (the periodic number) of a two-dimensional photonic crystal.
In patent literature 2, the two-dimensional photonic crystal is constituted of a first area and a second area. The first area is formed by two-dimensionally arranging refractive bodies with a first-order period for diffracting light to be emitted from an active layer within the photonic crystal. The second area is surrounded by the first area, and is formed by two-dimensionally arranging refractive bodies with a second-order period for diffracting light to be emitted from the active layer in a direction substantially perpendicular to a principal plane of the photonic crystal. In this way, disposing the first area around an outer periphery of the second area enables to trap light leaking from the second area into the first area by Bragg reflection.
Patent literature 3 discloses a two-dimensional photonic crystal, wherein a light reflecting area constituted of a low-refractive mirror is formed by e.g. forming an air-trapping groove in the perimeter of the crystal in a direction horizontal to a crystal plane. Thus, forming the light reflecting area in the periphery of the crystal in the direction horizontal to the crystal plane enables to reduce a likelihood that light may leak from the two-dimensional photonic crystal by Fresnel reflection.
In the technology disclosed in patent literature 2, it is necessary to increase the periodic number with respect to the first area to sufficiently trap light by Bragg reflection. Accordingly, although it is possible to decrease (narrow) the light emission area (the second area), it is difficult to decrease the size of a laser element itself, which corresponds to the sum of the first area and the second area.
In the technology disclosed in patent literature 3, since light is reflected by Fresnel reflection, it is difficult to completely reflect light. Thus, there is a limit in improving the performance of a resonator.
Further, neither patent literature 2 nor patent literature 3 recites or remotely suggests an influence of reflection light on light which is resonated within a two-dimensional photonic crystal.
Patent literature 1: JP 2000-332351A
Patent literature 2: JP2001-308457A
Patent literature 3: JP2003-273456A